The present invention relates to a method and a device for detecting a fault current across a piezoelectric actuator of an injector or its high voltage supply lead, according to the definition of the species of the main claim. In known methods, piezoelectric actuators are charged using a voltage of up to several hundred volts, for example to 200 volts, to control the switching valve or the nozzle needle of an injector. For service personnel this high voltage may represent a hazardous voltage source that can lead to electric shock upon direct contact with current-carrying parts or the supply lead. For example, during contact with the supply lead, a short circuit to ground can result in a current of an order of magnitude of 200 mA. Applicable safety regulations, such as VDE1 100, allow such a current for only a limited duration in order to prevent hazardous ventricular fibrillation. The measurement and detection of a fault current is not a simple matter, however, because in comparison the useful current of 20 to 30 A is several orders of magnitude higher, thus making the fault current difficult to detect.
1 Translator""s note: Verband Deutscher Elektrotechniker (Association of German Electrical Engineers) 
The method and the device according to the present invention for detecting a fault current across a piezoelectric actuator of an injector or its high voltage supply lead, having the characterizing features of the independent claims 1 and 11, in contrast, has the advantage that, by monitoring the voltage or the variation thereof across the charged actuator, even the relatively small fault current can be detected and protective measures can be effectively undertaken.
Advantageous developments of and improvements in the method described in the main claim derive from the measures represented in the subclaims. It is particularly advantageous that, in a fault scenario, a determined fault is weighted by a fault algorithm. Measurement errors or malfunctions, for example, are thus effectively suppressed, and only faults that actually occur are evaluated as such.
Furthermore, when an actual fault occurs, it is advantageous to shut off the voltage supply and also to discharge the actuator so rapidly that no contact hazard arises. In such a manner maximum protection is achieved and the danger of injury is largely avoided.
In a simple and particularly advantageous procedure, the voltage is determined at the beginning and at the end of the injection pause. Because the injection pause can last only several milliseconds, for example 2 ms, with the charged actuator, significant discharge of the actuator occurs during a short circuit.
For a system with multiple injection, it is also advantageous to perform voltage monitoring for individual injection pulses. The fault event can thus be detected early.
Since a plurality of actuators is generally supplied with high voltage from the power supply, it is advantageous to disconnect all actuators to increase safety.
Furthermore, it is advantageous to design the fault diagnosis in the form of a software program, since such a program can be created relatively easily. In addition, revisions can be made more readily.
The software program for fault diagnosis is advantageously incorporated into the control program for the actuator, thereby eliminating the need for additional hardware.